The goal of this project is to study how the pitch of complex periodic tones is coded at the earliest stages of the auditory nervous system. Pitch is an important attribute of human voice and most musical sounds, and is therefore important for communication and quality of life. We will approach this problem by recording the activity of single units in the auditory nerve and cochlear nucleus of anesthetized cats in response to a large set of complex-tone stimuli modeled after those used in psychophysical experiments. Some of these stimuli share a common periodicity, but produce different pitches, while others produce the same pitch even though their waveforms are very different on pitch. We will identify response features that may encode pitch over a broad range of stimulus conditions, and examine how the different theories of pitch of complex tones fit with our results. Specifically, we will examine under what conditions the average rates of discharge of auditory-nerve fibers increase when their characteristic frequency is a small integer multiple of the fundamental frequency, thereby providing a rate-place cue to the pitch of complex tones. We will also examine whether interspike intervals corresponding to the pitch appear prominently in the responses of auditory- nerve fibers and cochlear-nucleus units, particularly choppers. These responses will be interpreted in terms of a model of chopper neurons that simulates response properties likely to be important for pitch processing. This model, which complements existing ones in that it emphasizes intrinsic oscillatory behavior in nerve membranes rather than patterns of synaptic inputs, will be tested against data for individual units. Understanding the neural mechanisms of pitch processing may lead to improvements in the performance of hearing aids and cochlear implants for both speech and music.